Flatten image using Neural network and matrix transpose
Yes, if you have 3 images (and by images I assume you mean samples) the flatten layer will be of the shape $12288*3$ ($64*64*3=12288$). The size of $W$ however does not change, and nor does the size of $b$ as these are parameters and are independent of the amount of samples passed through the network.
ETA: I only answered the &quot;Am I right?&quot; part of your question because that's the only part of your questions that's actually a question. I don't know what you're trying to ask in the second half of your question